Bone is a complex and very important multi-constituent bio-composite. In this work, we focus on the arrangement of bone constituents from the nanoscopic to the microscopic scale, and investigate the influence of their arrangements on the fracture mechanisms of the whole composite. We find that bone, on the nanoscopic scale, consists of mineralized collagen fibrils held together by a non-fibrillar organic matrix, which results in a primary failure mode of delamination between mineralized fibrils. In turn, these mineralized fibrils form one of three types of filaments that span microcracks in fractured bone samples, possibly resisting the propagation of these cracks. (c) 2005 Elsevier Ltd. All rights reserved.